Happily enough, the essential fabrication steps for many types of flexible circuits—etching the circuit layers, adding a layer of metal for the wiring, and etching that to shape—can all be accomplished with the relatively cheap, and widely available, technologies used to print ink on paper. In fact, technologists are now working on several flexible-circuit manufacturing processes based on printing on paper. One is a modification of inkjet printing, the process at the heart of countless desktop printers [see photo, "Cut and Print"]. Another adapts roll-to-roll processing, a technique commonly used to print fabrics and newspapers. A roll of unprocessed plastic is put through a processing step and is rolled up again after the step is completed—like movie film passing through a projector.

Tantalized by the prospect of a new, multibillion-dollar market, inkjet-printing companies like Dimatix, in Santa Clara, Calif., and Litrex Corp., in Pleasanton, Calif., are developing processes that could replicate the basic steps of IC manufacturing—in machines not much bigger than desktop printers. How big is the commercial potential of an inkjet process for making circuits? Big enough to have inspired Dimatix recently to change its name from Spectra Inc., its headquarters from Lebanon, N.H., and its focus from printing posters to printing electronics.

In inkjet printing, tiny nozzles squirt droplets of ink onto a sheet of paper. There are two types of inkjet printers—those that use heat from resistors to form tiny bubbles to push the ink out of the nozzles and those that use a piezoelectric material inside each nozzle, which deforms when a voltage is applied. Under voltage, the piezoelectric material vibrates, pushing the ink out of the nozzle. These same mechanical vibrations pull more ink into the nozzle to replace the ink that was squirted out. The digital data for the document tells the piezoelectric material where and when to vibrate to get the drops in just the right places. In the heat method, the data sends current through the resistors to achieve the same result.

According to Dimatix scientists, only the piezoelectric process is suitable for making plastic electronic circuits, because the hot resistors used in the other method would damage both the polymers that make the transistors and the plastic substrates themselves. However, Hewlett-Packard is developing a process for printing circuits using the thermal process.

To print circuits instead of documents, Dimatix researchers replace the ink with a liquid containing an organic semiconductor or a metallic conductor. Today, inkjet-compatible liquids are also available for printing organic polymers to make the semiconducting components, and for laying down silver to make the connective wires. But at the moment no suitable liquid has been found for printing amorphous silicon with inkjets.

As you might expect, the printhead that Dimatix engineers developed for printing circuits is more complicated than the ones in desktop inkjet printers. There are more nozzles—over 100 on a circuit printer, compared with 30 on a desktop machine. Adding to the complexity, the direction in which each nozzle fires and the amount of ink in each drop are individually controlled—features not found on desktop printers.

These capabilities are needed to achieve the resolution and precision required for useful circuits. Today, the Dimatix jet heads can print lines and spaces 50 m wide with 5-m accuracy. Soon engineers expect to be able to bring line widths down to less than 10 mm.

In the Philips project, specialists at the Netherlands-based giant are working with Dimatix jet heads to print plastic organic-light-emitting-diode (OLED) displays for cellphones and other applications. By spraying solutions of red, blue, and green organic light-emitting material onto the display substrate, they have produced displays of up to 2 inches measured diagonally that are every bit as crisp and bright as displays made on glass substrates. Other researchers are using the inkjet technique to apply the color filters for LCDs.

Litrex Corp., which makes inkjet printers using Dimatix printheads, is also getting into circuit printing. It has already built inkjet printers for making flat-panel displays on both glass and plastic substrates. Including printers installed in R&D labs, Litrex has sold more than 50, mostly for making OLED arrays.

For making circuits, the failure rate of the inkjet nozzles must not be more than about a thousandth of the 1 percent or so allowed for printing documents. Litrex developers have achieved this level of reliability by including a high-speed camera in each printer that visually inspects the drops from each nozzle. It can capture images of single drops fired at a rate of up to 20 000 per second. The display substrate is loaded into the printer only after the inspection shows that all nozzles are firing correctly.

Litrex engineers are now testing a printer for large 2.4- by 2.4-meter substrates, now a standard size used by display makers. Although it is technically possible to build a single 2.4- by 2.4-meter display, most manufacturers build six separate displays within that area.

One important advantage of the inkjet print process over conventional techniques is that the jet process puts the circuit material only where it is needed, whereas the conventional process puts the material down over the whole substrate and then etches most of it away. But the fluids used to make electronics are pricey—up to US $10 000 per liter. A 7-inch-diagonal display printed conventionally might require a milliliter of these fluids, costing about $10. But an inkjet process would use only half a milliliter, saving the manufacturer $5. Multiply this by millions of displays, and it amounts to considerable savings.

At Motorola Inc., in Schaumburg, Ill., senior manager Daniel Gamota and his colleagues are taking printed electronics one step further. They are using conventional printing presses—the same ones that make posters and consumer product labels—to make circuits. These presses typically use metallic, rubber, or plastic cylinders 30 cm wide and 45 cm around, in which the patterns to be printed are etched.

Gamota and his team rent time on such printers from graphics arts companies and replace the standard printing inks with an assortment of electrically functional inks, which could be conducting, semiconducting, or insulating and organic or inorganic. So far they have produced more than 50 kilometers of circuitry, mostly timing and control circuits that switch at tens of hertz. These still-experimental circuits are too slow, even for displays. But they are fast enough to make electronically active labels for consumer packaging. So, for example, a timing circuit could switch on an indicator when a product reaches its expiration date. Or a sensor could detect when a package of food has spoiled.

Engineers are just starting to look into making flexible electronics with the roll-to-roll process. By eliminating the high-temperature, high-vacuum steps used in conventional circuit manufacture, it holds the promise of cutting the manufacturing cost by that magic factor of 10 or better.

The Fraunhofer Institute for Reliability and Microintegration, in Munich, Germany, has set up a laboratory to develop roll-to-roll processing of circuits on plastic. There, Karlheinz Bock and his group have produced thin-film transistors from organic semiconductors. They have measured individual transistors that switch at speeds up to 2 kilohertz. The first applications are likely to be low-tech and inexpensive electronics for things like radio-frequency identification tags and smart cards, though they can't say when the process will be commercially viable.